Often called the “evil twin” of climate change, ocean acidification results from fairly simple chemistry and yet has significant implications for marine species and ecosystems. While scientists have resolved the details of many of the physical and chemical processes related to ocean acidification, they have not yet built a parallel understanding of the future of marine ecosystems in a more acidic ocean. Acidification clearly interferes with the ability of some marine species to build shells and develop normally, but how do these changes in individuals translate to changes in populations, food webs, and entire ecosystems through both direct and synergistic processes? In this symposium, panelists highlighted research on the effects of ocean acidification on key ecological processes (e.g., food-web dynamics) in the ecosystems most vulnerable to changes in pH: polar regions, coral reefs, and temperate upwelling zones. From tiny genetic changes to existing ecosystem-wide shifts in species dynamics to glimpses into the ecological future via models, these scientists are on the front lines of asking if and how marine species can adapt to a rapidly changing environment. In short, science from the fields of oceanography, physiology, genetics, ecology, and modeling is starting to paint a picture that small changes in ocean chemistry can cause big shifts in the ability of marine ecosystems to deliver the goods and services on which we depend.